1. Field of the Invention
The present invention relates generally to consumer device audio power output stages, and more specifically, to a control circuit for selecting the power supply voltage of an audio power stage.
2. Background of the Invention
In battery-operated consumer audio devices, the power amplifier output, which is generally utilized to drive a pair of headphones or a speaker can be the primary consumer of battery power. In devices having a linear power amplifier for the output stage, power is wasted during low signal level outputs, since the voltage drop across the active output transistor plus the output voltage will be equal to the constant power supply rail voltage. Even though the current in the transducer is reduced at low signal amplitudes, more power is still wasted by dissipation in the output transistors than is delivered to the load. Battery power is typically only reduced in linear amplifier circuits by lowering the power supply voltage, which limits the available output power and also may not be possible in some applications in which the power amplifier output also serves as a line output signal that must provide a specified signal level at full signal level, for example +5 dBm (15 dB over −10 dBm) as referenced into a 600 ohm load, which requires a signal swing of 3 Vp-p.
Therefore, it would be desirable to provide a power amplifier circuit for a consumer audio device that has improved efficiency and reduced power dissipation, while maintaining a specified full-signal output level capability. It would further be desirable to provide control mechanisms within such an amplifier circuit that conserve power and reduce power dissipation while avoiding clipping of the audio signal.